1. Field of the Invention
The present invention relates to low noise block downconverters, and more particularly to a low noise block downconverter in a satellite receiver receiving satellite broadcasting.
2. Description of the Background Art
FIG. 7 is a block diagram showing a configuration of a conventional satellite broadcasting receiving system. Referring to FIG. 7, a high-frequency signal in the 12 GHz band (11.45-11.70 GHz) arriving from a broadcasting satellite 100 is received by an antenna 101. Antenna 101 is provided with a low noise block downconverter (hereinafter, referred to as “LNB”) 102. The weak high-frequency signal of the 12 GHz band from broadcasting satellite 100 is frequency-converted to an intermediate frequency (IF) signal of 1 GHz band and low-noise amplified by LNB 102. The resultant low-noise signal of a sufficient level is supplied to a digital broadcasting satellite (DBS) tuner 104 connected as a next stage. DBS tuner 104 processes the signal applied via a coaxial cable 103 in its internal circuit, and supplies the processed signal to a television receiver 105.
FIG. 8 is a block diagram of LNB 102 shown in FIG. 7. Referring to FIG. 8, the incoming signal in the 12 GHz band is received by an antenna probe 112 within a waveguide 111, and low-noise amplified by a low noise amplifier (LNA) 113. It is then passed through a band pass filter (BPF) 114, which allows a desired frequency band to pass and removes a signal in the image frequency band. Thereafter, the resultant signal is mixed by a mixer circuit 115 with a local oscillation signal of 10 GHz from a local oscillator circuit 116, and frequency-converted to an IF signal in the 1 GHz band (1450-1700 MHz). It is then applied to an intermediate frequency (IF) amplifier 117 to have appropriate noise and gain characteristics, and is output from an output terminal 118. LNA 113, local oscillator circuit 116, and IF amplifier 117 are provided with a power supply voltage from a power supply circuit 119.
The incoming signal from broadcasting satellite 100 has two kinds of polarized waves, i.e., H polarized wave and V polarized wave (or R polarized wave and L polarized wave). There is an LNB which frequency-converts the polarized waves separately from each other to prevent them from overlapping in the intermediate frequency band, and transmits them in one signal cable.
FIG. 9 is a block diagram showing such a band stack LNB. Referring to FIG. 9, a V polarized wave signal and an H polarized wave signal included in the incoming signal are received by a V polarized wave probe 122 and an H polarized wave probe 123, respectively, in a waveguide 121, and low-noise amplified by an LNA 124. The resultant signals are passed through respective BPF 125, 126, where desired frequency bands are passed and signals in the image frequency bands are removed. They are then applied to mixers (MIX) 127, 128, where the respective signals are mixed with local oscillation signals of 10 GHz and 10.45 GHz from local oscillator circuits 129 and 130, respectively, and converted to IF signals of 1000-1700 MHz.
The IF signal from MIX 127 and the IF signal from MIX 128 are passed through a high pass filter (HPF) 131 and a low pass filter (LPF) 132, respectively, such that they do not constitute interference components with respect to each other, and then mixed at a mixer circuit 133. The IF signal thus mixed is transmitted to and amplified at an IF amplifier 134, and is output from an output terminal 135. Power supply circuit 136 supplies power supply voltages to respective circuits.
It is now focused on LPF 132 in band stack LNB shown in FIG. 9. Since this LPF 132 needs to have attenuation characteristic that is wideband and steep and allows great attenuation in the reject band, multiple stages of series resonant trap circuits, each having an inductor L and a capacitor C connected in series as shown in FIG. 10, are connected as shown in FIG. 11 and employed.
When the series resonant trap circuits as shown in FIG. 11 are being employed, the number of stages must be increased in order to achieve the attenuation characteristic being wideband and steep and allowing great attenuation in the reject band, and inductor components La, Lb must be provided between the adjacent stages. Inductor component La, Lb would require an area when formed with a substrate pattern. It would increase the cost when a chip component is used.